Objective lens

ABSTRACT

An objective lens for focussing charged particles includes a magnetic lens and an electrostatic lens whose components are displaceable relative to each other. The bore of the outer pole piece of the magnetic lens exhibits a diameter D a  which is larger than a diameter D i  of the bore of the inner pole piece of the magnetic lens. The following relationship is satisfied: 1.5·D i ≦D a ≦3·D i . The lower end of the inner pole piece is disposed in a distance of at least 2 mm offset from the inner end of the outer pole piece in a direction of the optical axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12/551,783, filed Sep. 1, 2009, which is a continuation of, andclaims benefit under 35 USC 120 to, international applicationPCT/EP2008/001797, filed Mar. 6, 2008, which claims benefit of GermanApplication No. 10 2007 010 873.9, filed Mar. 6, 2007. U.S. applicationSer. No. 12/551,783 and International application PCT/EP2008/001797 arehereby incorporated by reference in their entirety.

FIELD

The disclosure relates to an objective lens for focussing chargedparticles and to a particle-optical system with such an objective lens.

BACKGROUND

Generally, in technology an accelerating tendency for miniaturizing canbe observed and the thereby evolving fields of micro- and nanotechnologyinvolve a demand for instruments that allow to manufacture such tinysystems and/or to inspect these. Especially the components andstructures in the field of semiconductor technology, becoming smallerand more complex, involve particle-optical systems for inspecting andmanufacturing the sub-micrometer sized functional components, whereinthese particle-optical systems ought to provide a sufficiently highresolution. For focussing the charged particles, magnetic lenses, aloneor in combination with electrostatic lenses, are conventionally used insuch particle-optical systems. The achieved resolution in such a systemsubstantially depends on the imaging properties of the system as awhole.

A known embodiment of a magnetic lens is referred to as “single pole”lens or also “Snorkel” lens and generally includes a single, usuallyconical pole piece which, exited by a coil, generates a magnetic fieldin the region of an object to be inspected.

Magnetic lenses with so-called axial pole piece gap are known as well.Such lenses usually include two pole pieces extending in a directiontowards an object plane of the objective lens. The inner diameters ofthe pole pieces are of equal size at their ends disposed closest to theobject. Thereby, an axial gap is formed arranged substantially parallelto an optical axis of the magnetic lens and the ends of the pole piecesare arranged at the same distance from the optical axis. Compared to thesingle pole (“Snorkel”) lens, for which the generated magnetic fieldextends a far amount into the space located in front of the objectivelens and thus includes the object, the magnetic lens with an axial polepiece gap has a characteristic steep decrease of the magnetic field.

Combination lenses formed of a magnetic lens and an electrostatic lensare known. In these types of lenses, the electrostatic lens may forexample be arranged in form of an immersion lens inside the magneticlens, such that electrostatic field and magnetic field overlap.

SUMMARY

In some embodiments, the disclosure provides an objective lens having amagnetic lens which exhibits good particle-optical properties andenables good resolution, even at small working distances.

In certain embodiments, the disclosure provides an objective lens havinga magnetic lens and an electrostatic lens which allows a satisfactoryadjustment of the magnetic lens and the electrostatic lens relative toeach other.

In some embodiments, the disclosure provides an objective lens forfocussing charged particles that includes a magnetic lens and anelectrostatic lens. The magnetic lens has an inner pole piece and anouter pole piece. A gap is formed between a lower end of the inner polepiece and an inner end of the outer pole piece. The electrostatic lenshas an electrode arrangement for generating an electrostatic field. Theelectrode arrangement includes a first electrode and a second electrodeoffset from each other in a direction traverse to the charged particles.The magnetic lens and the electrostatic lens are adapted to bedisplaceable relative to each other, such as in a direction transverseto the direction of traversal of the charged particles. The objectivelens further includes an actuator for changing a position of the firstelectrode and the second electrode of the electrostatic lens relative toa position of the magnetic lens.

The electrostatic lens and the magnetic lens may be at least partiallydisplaceable relative to each other.

The actuator may be adapted for changing a position of the first and/orthe second electrode relative to a position of the magnetic lens.

The actuator may be an active element for causing a position to change,such as a motor, or may also be a mediator of a driving force withoutactually providing the driving force by itself. Thereby, an externaldriving force may be transferred by the actuator to cause the changing aposition.

The charged particles may be electrons, ions, muons or any othersuitable particles.

In some embodiments, the inner and the outer pole pieces are disposed ata same side of an object to be processed or to be inspected. Between theinner and the outer pole piece a coil is arranged generating a magneticflux when an electric current is present.

Thereby an objective lens is provided which allows in an advantageousmanner an adjustment of the magnetic lens relative to the electrostaticlens. In contrast to conventional systems in which the electrostaticlens is often fixed to the magnetic lens, the magnetic lens and theelectrostatic lens disclosed herein are held displaceable relative toeach other so that both lenses may be adjusted relative to each other,as desired. Further, an actuator is provided to achieve the relativedisplacement in a precise and controlled manner. Providing an actuatorallows for automated displacement and therefore displacement during anoperation of the objective lens, such as while charged particlestraverse the objective lens.

In some embodiments, the magnetic lens as well as the electrostatic lensare each substantially rotationally symmetric around a respectiveoptical axis. By the adjustability of the objective lens such anaccurate alignment and correspondence of the optical axes of themagnetic and electrostatic lens can be achieved that during focussing noshift of image details, such as portions of an imaged object, occurs. Incontrast, in known objective lenses such a disturbing effect can arisein spite of substantial efforts with respect to aligning the magneticand the electrostatic lens relative to each other.

In some embodiments, an objective lens for focussing charged particlesincludes a magnetic lens and an electrostatic lens. The magnetic lenshas an inner pole piece and an outer pole piece. A gap is formed betweena lower end of the inner pole piece and an inner end of the outer polepiece. The electrostatic lens has an electrode arrangement forgenerating a focusing electrostatic field. The electrode arrangementincludes a first electrode and a second electrode offset from each otherin a direction of traversal of the charged particles. The inner polepiece and the outer pole piece of the magnetic lens are displaceablerelative to each other, such as in a direction transverse to thedirection of traversal of the charged particles. The first electrode andthe second electrode of the electrostatic lens are displaceable relativeto each other, such as in a direction transverse to the direction oftraversal of the charged particles. The objective lens further includesan actuator for displacing the inner pole piece and the outer pole piecerelative to each other and for displacing the first electrode and thesecond electrode relative to each other.

In certain embodiments, the first electrode is configured as a beam tubetraversing the inner pole piece. A gap between an outer circumferentialsurface of the beam tube and an inner circumferential surface of theinner pole piece is at least partly filled by an annular member, such asa tubular member, which can be made from insulating material, forholding the beam tube and the inner pole piece in a fixed positionrelative to each other.

In some embodiments, the objective lens further includes a connector forholding the first electrode and the inner pole piece in a fixed positionrelative to each other.

In certain embodiments, the actuator may for example be particularlyadapted to change at least the position of the second electrode relativeto the position of the magnetic lens. Further, the actuator may beadapted to change the position of the first electrode relative to theposition of the second electrode. In other embodiments also a furtheractuator may be provided. The actuator may further be designed to changea position of the inner pole piece relative to a position of the outerpole piece or adjust these positions.

Changing the relative position of the electrode/electrodes or therespective pole piece may be achieved substantially orthogonally to thedirection of traversal of the charged particles (orthogonally to theoptical axis of the magnetic or electrostatic lens), which may besubstantially in a radial direction. In particular, changing therelative positions may be in a direction transverse to the direction oftraversal of charged particles. Alternatively or additionally, changingthe relative position of the respective electrode or/and the respectivepole piece may occur substantially parallel to the direction oftraversal of the charged particles (in an axial direction). In exemplaryembodiments, the actuator allows an axial changing the position (aradial changing the position of a respective electrode or of arespective pole piece to be performed independently of each other).However, it is also conceivable that changing the positions areperformed in both directions concurrently. Also, a component may beprovided which synchronously causes an axial as well as a radialchanging the position/positions. The actuator may be designed to be asingle actuator or may also be designed to include plural, for examplealso independently acting or operable, actuators or actuator elements.

In exemplary embodiments, the second electrode is a terminal electrodecontinuing, in a direction towards an object plane of the objectivelens, from the outer pole piece in a direction of traversal of thecharged particles and is offset from the first electrode, in particularalong the same direction. The terminal electrode may for example includea disk having an aperture for traversal of the charged particles (anannular electrode). The terminal electrode may also be conicallyconstructed. Usually in particle-optical systems, the charged particlesare focussed at an object plane of the objective lens, which meansusually at a plane in which also the object to be inspected or to beprocessed is arranged. Herein, objective plane refers to the plane inwhich the object is arranged or in which the object is arrangeable, alsoin such embodiments, in which the objective lens is adapted, to focuscharged secondary particles to a detector. Thus, the detector could beconsidered to be the objective plane in optical respects. Accordingly,the terms “upper” and “lower” are to be understood as disposed fartheraway from the object plane (closer to the objective plane). Secondaryparticles are understood to include all particles generated byinteraction of primary particles generated by a source with an object,particles backscattered from the object and also reflected particles.

The first electrode may for example be configured as a beam tube atleast partially traversing the inner pole piece. In an exemplaryembodiment, a lower end of the beam tube may substantially lie in a sameplane as the inner end of the outer pole piece. In such an embodiment,an advantageous positioning of the electrostatic field generated by theelectrostatic lens relative to a magnetic field generated by themagnetic lens is achieved.

In an exemplary embodiment, the second electrode includes a portionextending from the outer pole piece towards the object plane.

The electrostatic lens allows deceleration of charged particles having ahigh acceleration voltage and thus having high kinetic energyimmediately before impinging onto the object plane by generating anelectrostatic retarding field. Thus, by using such a combination lens,significantly improved imaging properties may be achieved, for examplewith respect to chromatic and aperture errors.

In some embodiments, the objective lens includes a first sealing memberadapted and configured to prevent a pressure balancing via an interspacebetween a lower portion of the first electrode and the inner end of theouter pole piece; and a second sealing member adapted and configured toprevent a pressure balancing via an interspace between an upper end ofthe first electrode and the inner pole piece. Thus, a gas flow throughthe interspace between the upper end of the first electrode and theinner pole piece is prevented and a gas flow through the inter spacebetween the lower portion of the first electrode and the inner end ofthe outer pole piece is prevented. Each sealing member may be a singlesealing member or may be assembled from several components. The firstsealing member serves to prevent that a gas flow occurs between thelower part of the first electrode and the outer pole piece. Thereby, aninner space of the magnetic lens and a space between the first electrodeand the magnetic lens is sealed against a through-flow of gas. Thesealing member may for example displaceably directly be coupled to theouter pole piece and the first electrode or may contact these, and maybe configured as a sealing ring. Also, the sealing member may forexample be made from a concurrently isolating ceramic material or mayinclude such a material. The sealing members for example also includebellows and optionally bellows suitable for ultra high vacuum. Moreoverthe sealing members may include metal seals, metal bellows or othersuitable sealings which optionally are suitable for ultra high vacuum.

In some embodiments, a particle-optical system having an objective lens,such as with a first sealing member and a second sealing member, asdescribed above, is provided which further includes a vacuum chamberhaving a vacuum chamber wall for accommodating an object to be examined.The actuator, the first sealing member and the second sealing member arearranged such that the actuator and/or at least a handling element forhandling the actuator are disposed in a space separated from the vacuumchamber.

This particle-optical system thus allows in an advantageous way anadjustment of the electrostatic lens relative to the magnetic lensduring the operation of the particle-optical system (during thetraversal of charged particles through the objective lens).

The particle-optical system may for example be a microscope or alithographic system, or for example a lithographic system having anobservation mode. The system may be a scanning electron microscope. Themicroscope may further include a gas supply, to perform additive orsubstractive lithography using reactive gases in combination withcharged particles (to deposit material onto the object, or ablatematerial from the object) using the beam of charged particles.

In exemplary embodiments, the objective lens further includes anelectrode holder adapted to hold the first and optionally also thesecond electrode.

The electrode holder may for example include an insulating, for exampleceramic, element. The second electrode may suitably be fixed to thisinsulating ceramic element. The insulating ceramic element may at thesame time also serve as a sealing member and may for example beconfigured as a ring having an aperture for traversal of the chargedparticles. The insulating ceramic element itself may for example befixed to a suitable component of the objective lens (of theparticle-optical system) using a bracket or another suitable holdingmechanism. This component can be different from a component of themagnetic lens. The bracket or holding mechanism may for example extendapproximately parallel to an outer side of the outer pole piece. Theinsulating ceramic element may further also be fixed to a lower end ofthe first electrode such that a relative position of the first electrodeand the second electrode relative to each other may be fixed while it isconcurrently enabled to apply (different) potentials to the electrodesindependently. The insulating ceramic element may at the same timefunction as a first sealing member (part of a first sealing member).

Beside ceramic materials also any other suitable insulating materialsmay be utilized.

In exemplary embodiments, the electrode holder is displaceably heldrelative to the inner pole piece as well as relative to the outer polepiece. A changing the position of the second and possibly concurrentlyalso the first electrode includes a changing the position of theelectrode holder relative to the magnetic lens.

In an exemplary embodiment of a particle-optical system, the electrodeholder is vacuum tightly connected to the vacuum camber wall. In theconfiguration of the electrode holder, in particular also of theinsulating member of the electrode holder mentioned above, as a sealingmember, thus, a separation of the vacuum space defined within vacuumspace walls from a space in the region of the magnetic lens can beachieved. In this respect, also the second sealing member contributeswhich member separates the space accommodating the magnetic lens fromthe vacuum space inside the vacuum chamber and which member provides aconnection of an inside of the second electrode, such as of the beamtube, to a portion of the vacuum system of the particle-optical systemcontinuing in a direction opposite to a direction towards the objectplane.

The actuator may for example include a first actuator which isconfigured to shift the electrostatic lens, in particular the secondand/or the first electrode, relative to the outer pole piece. Such anactuator may for example be provided by a screw causing a radial shiftof the magnetic lens. A further actuator may include another screw whichis adapted and configured to radially shift the inner pole piecerelative to the outer pole piece. In exemplary embodiments, the changingthe position of the second electrode is achieved by a displacement ofthe electrode holder holding the second electrode.

There may be further actuators or actuator arrangements provided forcausing a positionally changing in at least one other direction, forexample in the axial direction.

Further, the actuator may include an actuator member which is adaptedand arranged to displace the second electrode relative to the firstelectrode. The actuator member may be configured to achieve a positionalchanging in an axial and/or radial direction.

In certain embodiments, an objective lens for focussing chargedparticles is provided that includes a magnetic lens having an opticalaxis, an inner pole piece and an outer pole piece. A gap is formedbetween a lower end of the inner pole piece and an inner end of theouter pole piece. The outer pole piece has a substantially conical shapehaving a diameter D_(a) at its inner end and wherein the inner polepiece has a diameter D_(i) at its lower end. The lower end of the innerpole piece is arranged at a distance H, when measured along a directionof the optical axis, apart from the inner end of the outer pole piece.Further, the inner end of the outer pole piece is a functional part ofthe magnetic lens disposed closest to an object plane of the objectivelens and the following conditions are fulfilled:1.5·D _(i) ≦D _(a)≦3·D _(i)and

-   -   25°≦α≦70°, in particular    -   30°≦α≦60°, more in particular    -   40°≦α≦50°, wherein        α=arc tan((D _(a) −D _(i))/(2*H)).

Thereby, a magnetic lens having a gap formed by the inner pole piece andthe outer pole piece is realized, wherein the gap is tilted relative tothe optical axis. This magnetic lens enables to generate a magneticfield having a particularly advantageous distribution, in particularrelative to an object to be inspected or to be processed.

In other words, the objective lens does not include any furtherfunctional parts which would displace the magnetic field generated bythe inner and the outer pole piece substantially compared to a magneticfield generated by the inner and outer pole piece only (in the absenceof further parts). In particular, the objective lens does not includeany further functional parts by which the shape of the magnetic fieldwould change as a consequence of adding them. Parts or componentsdisposed offset from the magnetic lens in a direction towards theobjective plane are substantially not magnetizable and do notsubstantially guide magnetic flux which is also guided through the outerpole piece. Components disposed offset from the magnetic lens in adirection towards the object plane may include for example components ofan electrostatic lens, such as electrodes, isolators and their holdingstructures. Substantially not magnetizable materials are for examplematerials which relative permeability μ_(i)≦1.001.

In exemplary embodiments, the lower end of the inner pole piece isdisposed, in a direction of the optical axis which at the same timedefines the direction of traversal of the charged particles through themagnetic lens, in a distance of at least 2 mm, in particular 3 mm, fromthe inner end of the outer pole piece. This distance is also denoted asH. In further exemplary embodiments, H is at most 10 mm.

In exemplary embodiments, a distance between the lower end of the innerpole piece and the first electrode is 7.5 mm or more.

These distances refer to distances between the respective lowest(disposed closest to the objective plane) surfaces of the respectivepole piece ends or of the electrode.

Further, it is advantageous to arrange the inner pole piece and theouter pole piece such that they are displaceable relative to each other.In advantageous embodiments, the objective lens in accordance withfeatures of embodiments described above, includes a first actuator thatis configured to change a position of the inner pole piece relative to aposition of the outer pole piece.

Further, the objective lens includes in exemplary embodiments anelectrostatic lens having an electrode arrangement for generating anelectrostatic field, wherein the electrode arrangement includes a firstelectrode and a second electrode offset from each other in a directionof the optical axis.

Such a combination lens formed by a magnetic lens having an angular(tilted) pole piece gap and an electrostatic lens provides particularadvantages. In particular, such a combination lens allows for a finefocussing of the charged particles in the objective plane also for highacceleration voltages of the charged particles and also for smallworking distances. In particular, for small working distances the errorcoefficients of such a combination lens are smaller than for largeworking distances so that an improved resolution can be achieved.Compared to conventional combination lenses having an axial pole piecegap, the different positions of the magnetic field relative to theobjective plane and relative to an electrostatic field generated by theelectrostatic lens proves to be advantageous, since by a better overlapof the magnetic and electrostatic field smaller error coefficients areachieved and therefore better resolutions can be provided.

The small working distances also provide for high acceleration voltagesthe advantage that negative effects of perturbation fields occurringduring an operation of particle-optical systems are smaller than forlarge working distances.

In an exemplary embodiment, the objective lens includes a secondactuator adapted to change a position of the first electrode relative tothe position of the second electrode and/or relative to the position ofthe outer pole piece. First and second actuators may be parts of a sameactuator. Regarding possible embodiments of the actuators or actuatormembers it is referred to the description in the context of embodimentsof the disclosure provided above.

In exemplary embodiments, the first electrode is a beam tube at leastpartially traversing the inner pole piece and the second electrode is aterminal electrode disposed offset from the beam tube and continuing, ina direction towards the object plane, from the outer pole piece towardsthe object plane. Further, the first actuator is adapted to displace theinner pole piece and/or the outer pole piece relative to the firstand/or the second electrode of the electrode arrangement. Also severalcombinations regarding the configuration and functioning of the actuatoror the actuators may be provided, as described further above.

Furthermore, the disclosure provides a particle-optical system having anobjective lens according to some embodiments. The particle-opticalsystem may include the features of embodiments of the particle-opticalsystem described above.

A particle-optical system can have a separation of the vacuum space inthe vacuum chamber from the space to accommodate the magnetic lens,which can enable in an advantageous manner disposing the actuator and/orat least a handling element for handling the actuator outside the vacuumand thus in a space in which atmospheric pressure may prevail.

A particle-optical system thus allows for changing the relativepositions of the pole pieces relative to each other and/or changing theposition of the magnetic lens relative to the position of theelectrostatic lens during operation of the objective lens (theparticle-optical system). Thus, good imaging properties of the objectivelens can be achieved.

Typically, the particle-optical system includes, besides an objectivelens, a source for generating charged particles, for example forgenerating a beam of charged particles, and an objective holder fordisposing an object to be examined or to be processed in a desiredposition relative to the objective lens. Further, a particle-opticalsystem may include a condenser lens and/or deflecting elements fordeflecting the beam of charged particles. Particle-optical systems forinspecting further include one or plural detectors which are arranged ina region of the vacuum chamber or which are arranged as so-calledin-lens detectors within the particle-optical system above the objectivelens.

In some embodiments, the disclosure provides a method for adjusting(changing) the relative positions of the pole pieces of the magneticlens or changing the magnetic lens relative to the electrostatic lens.The method may include the following. A source of electrostaticparticles emits charged particle such that they are accelerated towardsthe object plane (towards the object) using an acceleration voltage ofmore than 5 kV. In a first method step low potentials are applied to theelectrodes of the electrostatic lens. In this method step, the objectivecurrent (currents flowing through the excitation coil of the magneticlens) is wobbled (varied to higher and lower values around an averagecurrent). During this wobbling the positions of the inner and outer polepieces are shifted relative to each other until a center of an imagegenerated by the charged particles in the objective plane movesminimally.

In a second method step a potential difference is applied across thefirst and the second electrode of the electrostatic lens. Using themagnetic lens, the charged particles are focussed to a working distancewhich is smaller than the distance corresponding to the focal point ofthe pure electrostatic lens having the potential difference applied. Theworking distance denotes the distance between the objective lens and theobject (the objective plane). In this method step, exclusively theacceleration voltage with which the charged particles are acceleratedtowards the objective plane is wobbled. Using the actuator now themagnetic lens (commonly the inner and outer pole piece) is shiftedrelative to the electrostatic lens, until the center of the image doesnot move anymore, but instead only a transition from an overfocus to anunderfocus occurs. This method step can be performed at accelerationvoltages between 100 V and 5 kV, since in this voltage range the effectof the electrostatic lens dominates over the effect of the magneticlens.

In the embodiments in which the actuator is also configured forpositionally changing the second relative to the first electrode, suchas where an additional actuator is provided for this purpose, a courseof an adjusting may be carried out, for example according to thefollowing method.

In a first method step, no potential difference is applied to the firstand second electrode of the electrostatic lens (no electrostatic fieldis generated). The charged particles are accelerated in a directiontowards the objective plane through the objective lens using anacceleration voltage of more than 5 kV. In this step the adjusting themagnetic lens is carried out by wobbling (by a continuous variation ofthe strength of the objective current, which means of the currentflowing through the excitation coil of the magnetic lens) and shiftingthe inner and outer pole pieces relative to each other until the movingof the image center is minimized, in a manner described already above.

In a second step, the magnetic lens is switched off (a current throughthe excitation coil of the magnetic lens is switched off). A potentialdifference between the first and the second electrode of theelectrostatic lens is applied, for example a potential of +8 kV may beapplied to the first electrode and the second electrode may be grounded.Consequently, the object to be inspected is disposed in the focus ofmerely the electrostatic lens. For adjusting, the potential applied tothe first electrode is subsequently wobbled and the second electrode isshifted relative to the first electrode, until a center of the imagegenerated by the charged particles does not move anymore and transitsonly from an overfocus to an underfocus. In alternative embodiments ofthis step, also the acceleration voltage could be wobbled, while thepotential applied to the first electrode is held constant.

In a third step, the object is disposed in a working distance which issmaller than the focal length of the electrostatic lens having appliedthe corresponding potential difference. The charged particles arefocussed using the magnetic lens according to this smaller workingdistance. In the following, the acceleration voltage of the chargedparticles is wobbled. In this method step, the magnetic lens is shiftedrelative to the electrostatic lens until the image center does not moveanymore but transits only from an over- to an underfocus. This step canbe carried out at acceleration voltages between 100 V and 5 kV, since inthis voltage range the effect of the electrostatic lens dominates overthe effect of the magnetic lens.

BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing as well as other advantageous features of the disclosurewill be more apparent from the following detailed description ofexemplary embodiments of the with reference to the accompanyingdrawings. It is noted that not all possible embodiments exhibit each andevery, or any, of the advantages identified herein.

FIG. 1 is a schematic illustration of an objective lens;

FIG. 2 is an objective lens;

FIG. 3 is an objective lens;

FIG. 4 is a simplified, schematic view of a particle optical systemhaving an objective; and

FIG. 5 is an illustration of graphs of intensities of the magnetic andelectric fields generated by the objective lens illustrated in FIG. 1along the optical axis.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the exemplary embodiments described below, components that are alikein function and structure are designated as far as possible by alikereference numerals. Therefore, to understand the features of theindividual components of a specific embodiment, the descriptions ofother embodiments and of the summary of the invention should be referredto.

In FIG. 1 a first exemplary embodiment of an objective lens according tothe present invention is illustrated. The objective lens includes amagnetic lens including an inner pole piece 1 having a lower end 1′ andan outer pole piece 2 having an inner end 2′. Between the lower end 1′of the inner pole piece 1 and the inner end 2′ of the outer pole piece 2an angular (tilted with respect to an optical axis OA of the objectivelens 100) pole piece gap 3 is formed. In an inner space 4 enclosed bythe inner pole piece 1 and the outer pole piece 2 a coil 5 is arrangedand provided to generate a magnetic flux in the inner and the outer polepieces 1, 2. When an electric current flows through the coil 5, amagnetic field is generated within the inner and the outer pole pieces1, 2 that substantially protrudes from the pole piece gap 3 in theregion around the pole piece gap 3. The inner pole piece and the outerpole piece guide the magnetic field excited by the coil 5 andconcentrate it within the pole pieces. The pole pieces oppose each othervia the pole piece gap 3 such that the magnetic field is established inthe pole piece gap having a predetermined strength and direction.Charged particles accelerated by a not illustrated source in a directiontowards the objective plane 102 traverse the objective lens 100 in thedirection of traversal 101 and thus traverse the magnetic fieldgenerated by the magnetic lens and are focussed at the objective plane102. The inner pole piece 1 exhibits a cylinder formed section throughwhich the charged particles traverse. The outer pole piece exhibits aconical section tapering in the direction towards the objective plane102.

The objective lens 100 further includes an electrostatic lens. Theelectrostatic lens includes a beam tube 10 as a first electrode. Thebeam tube is arranged in the inside of the cylinder shaped section ofthe inner pole piece 1 and disposed with a distance apart from thelatter and extends almost up to its upper edge. The lower end of thebeam tube 10 is configured as a flange 11 extending radially outwards.Further, the electrostatic lens includes a terminal electrode 12 as asecond electrode continuing in the direction of traversal of the chargedparticles from the outer pole piece 2, respectively from its end 2′(disposed closer to the objective plane 102 than the inner end 2′ of theouter pole piece 2).

The beam tube 10 and the terminal electrode 12 are held in position viaan electrode holder 13 and a first sealing member 20 as well as a firstconnection piece 16. The electrode holder 13 includes a conical holdingmember extending parallel to an outer face of the outer pole piece andis vacuum tightly connected to a component different from the magneticlens, such as for example a vacuum chamber boundary member 14 of aparticle-optical system including the objective lens 100. The firstsealing ring 20 includes a ring, such as a disk provided with an openingmade of insulating ceramics, and substantially extends radially betweenthe lower end of the electrode holder 13 and the lower end of the beamtube 10, where it is suitably fixed. The terminal electrode 12 ismounted to the first sealing ring 20 using the first connection piece16. Thereby, a fixing of the first and second electrodes 10, 12 relativeto each other as well as an electric insulating the first electrode 10from the second electrode 12 is effected. A voltage source for applyingpotentials to the first electrode and second electrode (to the beam tube10 and the terminal electrode 12) is not illustrated in FIG. 1 to notobscure the drawing. The outer pole piece 2 exhibits in its lowerportion a conical shape that transits at its upper end to an L-shapedirected outwards. An upper edge of this L-shaped portion is fixed tothe vacuum chamber border member 14 via a connecting piece 15. Further,at this L-shaped portion a third sealing 21 is mounted providing avacuum tight sealing between the outer pole piece 2 and the vacuumchamber border member 14. The third sealing 21 is in the presentexemplary embodiment configured as a diaphragm bellows. A furtherdiaphragm bellows is mounted as second sealing 22 at an upper end of thebeam tube 10 and provides a vacuum tight connection to a component 103continuing upwards which may be a component of the particle-opticalsystem including the objective lens 100. The component 103 may forexample be an electrode or a diaphragm or any other component to whichmay be suitably sealed. Thus, by the second sealing 22 a second sealingmember for the particle-optical system is provided. By the electrodeholder 13, the sealing 20 and the third sealing 21 commonly a firstsealing member for the particle-optical system is provided preventingtraversal of gas through an interspace between the outer pole piece 2and the beam tube 10.

The inner pole piece 1 substantially exhibits a L-shape, wherein theshort side of the L is directed radially outwards. The inner pole piece1 exhibits a plane face 1S (at the short side) abutting on a plane face2S of the outer pole piece 2. By these abutting, relative to each otherdisplaceable plane faces 15 and 2S a displaceable mounting of the twopole pieces 1 and 2 is achieved. To be able to achieve a changing aposition of the inner pole piece 1 relative to a position of the outerpole piece 2, the objective lens 100 includes a first actuator 31. Theactuator 31 includes a screw fixed to the outer pole piece 2 using afixing piece 18, wherein an end of the screw 31 contacts an outer edgeof the inner pole piece 1. Thus, rotating the screw 31 causes a radialdisplacement of the inner pole piece 1 relative to the outer pole piece2. The actuator 31 is adapted to change the relative positions of thepole piece 1 and the pole piece 2 in a direction transverse to thedirection 101 of traversal of the charged particles through theobjective lens 100. Further, actuators not illustrated in the sectionalview of FIG. 1, are provided to change the relative positions of theinner pole piece and the outer pole piece in another directiontransverse to the direction 101 of transversal of the charged particlesthat is substantially orthogonal to the direction of changing thepositions described above. The rotating of the screw 31 may be performedby an external driving mechanism, such as a motor. The objective lensfurther includes a second actuator 32, a screw 32 in the illustratedembodiment, fixed to the vacuum chamber border member 14 via aconnecting piece 17. The end of the screw 32 contacts an outer edge ofthe outer pole piece 2 so that rotating the screw 32 causes adisplacement of the inner pole piece 1 and the outer pole piece 2relative to the electrostatic lens having the beam tube 10 and theterminal electrode 12. Thus, a adjusting the optical axis of theelectrostatic lens and the optical axis of the magnetic lens relative toeach other is enabled so that both optical axes coincide and inparticular good imaging properties of the objective lens may beprovided. The coinciding optical axes are illustrated in FIG. 1 as acommon optical axis OA.

In the exemplary embodiment illustrated in FIG. 1 the inner pole pieceexhibits in the inner space traversed by the charged particles (in itscylindrical section) a constant diameter D_(i) of 15 mm. The diameter ofthe inner end 2′ of the outer pole piece has a diameter D_(a) amountingto 30 mm. Thus, the diameter D_(a) of the inner end 2′ of the outer polepiece 2 is two times the diameter D_(i) of the inner pole piece 1. Adistance H between the lower end 1′ of the inner pole piece 1 and theinner end 2′ of the outer pole piece 2 amounts to 4 mm in theillustrated embodiment.

This configuration of the magnetic lens having an inner pole piece andan outer pole piece leads to the property that the magnetic fieldcompared to conventional objective lenses protrudes further into thespace in front of the lens so that it overlaps with the electrostaticfield of the lens in an advantageous way. This is exemplarily apparentfrom FIG. 5 in which normalized intensities of the magnetic field andthe electric field are illustrated in dependence of a position along theoptical axis. A distance between the maximum of the magnetic field andthe maximum of the electric field amounts in the illustrated example toabout 3.7 mm. Such an overlap of the electric and magnetic fields causessmaller error coefficients of the objective lens and thus leads to abetter resolution, in particular in a larger range of working distances.A direction towards positive values along the x-axis in FIG. 5representing positions in units mm along the optical axis OA correspondsto a direction from the objective plane 102 towards the objective lens100. The objective plane 102 is located at negative values in FIG. 5 ataround −2 mm to −5 mm.

The configuration of the magnetic field generated by the geometry of theinner pole piece and the outer pole piece is substantially not effectedby the further components disposed in the region of the pole piece gapor disposed in front of this pole piece gap in a direction towards theobjective plane, such as the components 13, 16, 20 and 11, since thesecomponents are made of materials having relative permeability of forexample μ_(r)≦1.001.

In FIG. 2 a second exemplary embodiment of an objective lens 100according to the present invention is illustrated. This exemplaryembodiment differs from the embodiment illustrated in FIG. 1 mainly inthat the terminal electrode provided as the second electrode 12 isdisplaceably arranged relative to the beam tube 10. Furthermore, theconstruction of the electrode holder 113 and the first sealing ring 120is modified compared to the corresponding elements in FIG. 1. Theelectrode holder 113 in this exemplary embodiment extends substantiallyparallel to an outer face of the outer pole piece 2 up to a lower end113′. This lower end 113′ exhibits a larger inner diameter than theinner end 2′ of the outer pole piece 2. The lower end 113′ extendssubstantially orthogonally to the object plane 102. The lower end 113′of the electrode holder 113 is formed in an annular shape. The sealingring 120 configured, compared to the first exemplary embodiment, largerextends from the annular lower edge 113′ of the electrode holder 113 tothe lower end of the beam tube 10 and abuts the terminal flange 11 ofthe beam tube 10. Thus, in this exemplary embodiment, a vacuum tightsealing between the electrode holder and the beam tube 10 is provided sothat traversal of gas (a pressure balancing) via an interspace formedbetween the terminal flange 11 and the inner end 2′ of the outer polepiece is prevented. The terminal electrode 12 is held at the firstsealing ring 120 using brackets 123. Further, a third actuator 33 isprovided enabling a mechanical displacement of the terminal electrode 12relative to the beam tube 10. This third actuator may for example be apiezo-element. The first and the second actuator may also bepiezo-elements.

Diameter D_(i) of the inner pole piece 1 and diameter D_(a) of the innerend 2′ of the outer pole piece 2 are the same as these in FIG. 1.Further, in this FIG. 2, a diameter D_(E1) of the beam tube 10 and adiameter D_(E2) of an aperture enclosed by the terminal electrode areillustrated. These diameters amount in the illustrated embodiment toD_(E1)=4.2 mm and D_(E2)=5 mm. The remaining components of this secondexemplary embodiment illustrated in FIG. 2 correspond to these describedwith reference to FIG. 1.

Also illustrated in FIG. 2 is a line 1 touching the lower end of theinner pole piece 1 at a point 11 and touching the inner end of the outerpole piece at a point 21. The line 1 intersects the optical axis OA,wherein the line 1 and the optical axis OA include an angle α. The angleα characterizes an amount of a tilting of the pole piece gap formedbetween the lower end of the inner pole piece 1 and the inner end of theouter pole piece 2 relative to the optical axis OA of the objectivelens. A tangent of this angle α is the ratio between half of thedifference between the diameter D_(a) and the diameter D_(i) and thedistance H between the lower end of the inner pole piece 1 and the innerend of the outer pole piece 2. In the illustrated embodiment the angle αamounts to about 45°. In other exemplary embodiments of the presentinvention α may assume other values such as 30°, 40°, 50°, 60°.

In the exemplary embodiment illustrated in FIG. 2 the fixing piece 18illustrates a portion of a vacuum chamber wall of a particle-opticalsystem. Thus, the objective lens 100 illustrated in FIG. 2 may befixedly mounted to the vacuum chamber wall of the particle-opticalsystem via the outer pole piece 2 of the magnetic lens. Thereby theinner pole piece 1 and, via the inner space 4 between the inner polepiece 1 and the outer pole 2, also the outer pole piece 2 may be exposedto atmospheric pressure, while the vacuum chamber wall surrounds theobject plane 102 to maintain a vacuum in the object region.

The objective lens 100 fixedly mounted within a vacuum chamber of aparticle-optical system can then be adjusted using the actuators 31 toadjust an inclination of the optical axis of the magnetic lens, theactuator 33, to adjust an inclination of the optical axis of theelectrostatic lens, and the actuator 17 to adjust a relative position ofthe optical axis of the magnetic lens and the optical axis of theelectrostatic lens.

Other ways for mounting the objective lens 100 according to the presentinvention to a vacuum chamber of a particle-optical system areenvisaged. For example, instead of mounting the objective lens 100 viathe outer pole piece 2 and the connecting piece 18 to a chamber wall ofa vacuum chamber of a particle-optical system, the objective lens 100may be fixedly mounted to a vacuum chamber wall of the vacuum chamber ofthe particle-optical system by fixedly mounting for example the beamtube 10 to the chamber wall of the vacuum chamber of theparticle-optical system. In this case the connecting piece 18 is notfixedly connected to the chamber wall of the vacuum chamber of theparticle-optical system. Instead of mounting the objective lens via theconnecting piece 18, the objective lens can also be mounted via thevacuum chamber border member 14 to the particle optical system.

FIG. 3 illustrates a further exemplary embodiment of an objective lens100 according to the present invention. The exemplary embodimentillustrated in FIG. 3 is similar to the exemplary embodiment in FIG. 2.The two exemplary embodiments, however, differ in that in the embodimentillustrated in FIG. 3 the electrostatic lens can not be displacedrelative to the magnetic lens as a whole. Thus, in this exemplaryembodiment, the actuator 17 is not present. Instead, the beam tube 10and the inner pole piece 1 are held in a fixed positional relationshipby interposing an insulating material 10′ between an outercircumferential face of the beam tube 10 and the inner cylindrical faceof the inner pole piece 1. Since a relative displacement of the magneticlens relative to the electrostatic lens is not enabled in the exemplaryembodiment illustrated in FIG. 3, the electrode holder 113 is notneeded. The exemplary embodiment illustrated in FIG. 3 of the objectivelens 100 allows in an analogous way as the exemplary embodimentillustrated in FIG. 2 adjusting a position of the first electroderelative to a position of the second electrode via the actuator 33 andadjusting a position of the outer pole piece 1 relative to a position ofthe inner pole piece 2 via the actuator 31. As mentioned in the contextof the description of the exemplary embodiment illustrated in FIG. 2,there are several opportunities to fixedly mount the objective lens 100illustrated in FIG. 3 to a vacuum chamber wall of a vacuum chamber of aparticle-optical system. For example, the fixedly mounting may beeffected via the connecting piece 18 being fixedly mounted to the outerpole piece or it may be effected by fixedly mounting for example thebeam tube 10 to the chamber wall of the vacuum chamber of theparticle-optical system.

However, such mounting can also be carried out as illustrated in FIG. 3,wherein the sealing ring 120 extends from a lower end of the beam tube10 substantially parallel to the objective plane 102 in a radialdirection outwards up to a portion 18′ of a chamber wall of a vacuumchamber of a particle-optical system and is fixedly mounted thereto. Thekind of mounting the objective lens 100 within a particle-optical systemmay depend on the particular application.

FIG. 4 illustrates an exemplary embodiment for a particle-optical systemaccording to the present invention in which the objective lens 100according to the first exemplary embodiment is used. Theparticle-optical system of this exemplary embodiment is adapted as anelectron microscope. The typical components of such an electronmicroscope are well-known in the art and thus will only briefly bementioned in the following.

The electron microscope includes an electron source 53 depicted veryroughly. The electron source 53 includes a tungsten wire 52 which emitselectrons in a direction 101 extracted by an electric acceleration fieldtowards an anode. The electron source 53 is arranged in a vacuum space48 which is partly separated from a subsequent vacuum space 46 by adiaphragm 46 a. Vacuum space 48 includes a connection 49 to a notillustrated vacuum pump. In vacuum space 46 a schematically indicatedcondenser lens 51 is illustrated. Vacuum space 46 includes a connector47 to a not illustrated vacuum pump. Vacuum space 46 is at least partlyseparated from the subsequent vacuum space 44 by diaphragm 44 a. Invacuum 44 a detector 60 is disposed. The detector is a conventionalelectron detector which due to its location inside of the electronmicroscope is also referred to as inlens detector. The vacuum space 44is connected to a not illustrated vacuum pump via a connector 45. Thevacuum space 44 is vacuum tightly connected to the upper end of the beamtube 10 by the connecting member 103 and diaphragm bellow 22. Thus, viasealing ring 20 and sealings 21 and 22 a separation of a vacuum chamber40 from a space 43 is achieved in which space the magnetic lens and thefirst and second actuators are arranged.

The first and second actuators 31, 32 may be controlled using handlingmember 34. Thereby, and by the positioning of the displaceable parts ina space not under vacuum, it is possible to perform adjusting thecorresponding components relative to each other externally controlledand during the operation of the electron microscope.

The vacuum chamber 40 is bounded by vacuum chamber boundary members 14and 41. The vacuum chamber 40 exhibits a connector 42 to a vacuum pump.In the vacuum chamber 40 a object holder 70 is arranged which isconfigured and arranged to hold an object O and, as desired, to displacethe object in different directions. A surface of the object O can bearranged in the object plane 102. Also not illustrated in this Figurefor simplicity are a current source for supplying the coil 5 as well asa voltage source for applying potentials to the first and/or the secondelectrodes 10, 12. The electron microscope may also include deflectingmembers for deflecting the electron beam and further components.

While the invention has been described with respect to certain exemplaryembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, the exemplary embodiments of the invention set forth hereinare intended to be illustrative and not limiting in any way. Variouschanges may be made without departing from the spirit and scope of thepresent invention as defined in the following claims.

1. An objective lens, comprising: a magnetic lens having an opticalaxis, the magnetic lens comprising an inner pole piece and an outer polepiece, wherein a gap is formed between a lower end of the inner polepiece and an inner end of the outer pole piece, wherein the outer polepiece has a substantially conical shape having a diameter D_(a) at itsinner end, and the inner pole piece has a diameter D_(i) at its lowerend, wherein the lower end of the inner pole piece is arranged at adistance H, when measured along a direction of the optical axis, apartfrom the inner end of the outer pole piece, wherein the inner end of theouter pole piece is a functional part of the magnetic lens disposedclosest to an object plane of the objective lens, wherein the inner polepiece and the outer pole piece are displaceable relative to each other,and wherein the following conditions are fulfilled:1.5·D _(i) ≦D _(a)≦3·D _(i)and25°≦α≦70° where α=arc tan((D_(a)−D_(i))/(2*H)).
 2. The objective lensaccording to claim 1, wherein 30°≦α≦40°.
 3. The objective lens accordingto claim 1, wherein H is at least 2 mm.
 4. The objective lens accordingto claim 1, wherein H is at most 10 mm.
 5. The objective lens accordingto claim 1, further comprising a first actuator configured to change aposition of the inner pole piece relative to a position of the outerpole piece.
 6. The objective lens according to claim 5, furthercomprising an electrostatic lens comprising an electrode arrangementconfigured to generate an electrostatic field, wherein the electrodearrangement comprises a first electrode and a second electrode offsetfrom each other in a direction of the optical axis.
 7. The objectivelens according to claim 6, further comprising a second actuator adaptedto change a position of the first electrode relative to a position ofthe second electrode and/or relative to the position of the outer polepiece.
 8. The objective lens according to claim 6, wherein the firstelectrode is a beam tube at least partially traversing the inner polepiece and the second electrode is a terminal electrode disposed offsetfrom the beam tube and continuing, in a direction towards the objectplane, from the outer pole piece towards the object plane.
 9. Theobjective lens according to claim 6, wherein the first actuator isadapted to displace the inner pole piece and/or the outer pole piecerelative to the first and/or the second electrode of the electrodearrangement.
 10. The objective lens according to claim 6, wherein H isat least 2 mm, and H is at most 10 mm.
 11. The objective lens accordingto claim 10, one of the following conditions holds:30°≦α≦40°,40°≦α≦50°, or50°≦α≦60°.
 12. A system, comprising: an objective lens according toclaim 11, wherein the system is an electron microscope.
 13. A system,comprising: an objective lens according to claim 10, wherein the systemis an electron microscope.
 14. The objective lens according to claim 6,one of the following conditions holds:30°≦α≦40°,40°≦α≦50°, or50°≦α≦60°.
 15. A system, comprising: an objective lens according toclaim 14, wherein the system is an electron microscope.
 16. A system,comprising: an objective lens according to claim 6, wherein the systemis an electron microscope.
 17. The objective lens according to claim 1,wherein 40° a 50°.
 18. The objective lens according to claim 1, wherein50°≦α≦60°.
 19. The objective lens according to claim 1, wherein H is atleast 2 mm, and H is at most 10 mm.
 20. A system, comprising: anobjective lens according to claim 1, wherein the system is an electronmicroscope.